1. Field of the Invention
This invention generally relates to a semiconductor device, and more particularly to a protection diode structure for protecting an insulated gate type semiconductor device.
2. Description of the Related Art
FIG. 1 shows an n-channel MOS transistor as an example of an insulated gate type semiconductor device. The MOS transistor has n-type source and drain regions 4 and 5 formed in the surface area of p-type semiconductor substrate 1, oxide film 6 formed on that portion of semiconductor substrate 1 which lies between source and drain regions 4 and 5, and gate electrode 7 formed on oxide film 6. In the operating condition, a drive voltage is applied between gate electrode 7 and each of semiconductor substrate 1 and source region 4 to set the potential of gate electrode 7 higher than those of semiconductor substrate 1 and source region 4 so that an inversion region is created as a channel in the surface area of semiconductor substrate 1 between source and drain regions 4 and 5. The gate withstand voltage of the MOS transistor is set to be equal to the maximum voltage at which oxide film 6 can withstand the rupture. In a case where the drive voltage is set less than 20 volts, oxide film 6 is set to a thickness of, for example, approximately 500 .ANG. such that a gate withstand voltage of approximately 50 V can be obtained.
It is generally known that such a MOS transistor is easily affected by static electricity. In most cases, electrostatically generated voltages are higher than the gate withstand voltage. Therefore, the rupture of oxide film 6 may be caused by the voltage applied thereto and the MOS transistor will be destroyed permanently. PN junction diodes D1 and D2 shown in FIG. 1 are serially connected between gate electrode 7 and semiconductor substrate 1 in order to protect the MOS transistor from permanent destruction. PN junction diodes D1 and D2 are forwardly biased and reversely biased respectively in a case where the potential of gate electrode 7 is set higher than that of semiconductor substrate 1, and reversely biased and forwardly biased respectively in a case where the potential of gate electrode 7 is set lower than that of semiconductor substrate 1. When a voltage higher than the withstand voltage of the MOS transistor is applied between gate electrode 7 and semiconductor substrate 1, one of the PN junction diodes which is reversely biased is temporarily broken down. At this time, a breakdown current flows via PN junction diodes D1 and D2 between gate electrode 7 and semiconductor substrate 1, thereby protecting oxide film 6 from rupture.
The input capacitance of the MOS transistor is determined by a stray capacitance of series-connected PN junction diodes D1 and D2 and a capacitance between gate electrode 7 and semiconductor substrate 1. Since the capacitance between gate electrode 7 and semiconductor substrate 1 is in parallel relation with respect to the stray capacitance of PN junction diodes D1 and D2, the input capacitance of the MOS transistor increases by adding PN junction diodes D1 and D2. Increasing in the capacitance of the MOS transistor increases noise factor NF and lowers the operation speed.
In order to solve the above problem, it is considered to reduce the PN junction area of the PN junction diode, for example. However, when the PN junction area is reduced, the withstand voltage of the PN junction diode is undesirably lowered. Further, it is also considered to sufficiently reduce the impurity concentration of one of p- and n-type semiconductor layers constituting the PN junction diode. In this case, the breakdown voltage of the PN junction diode is raised, reducing the margin between the breakdown voltage of the PN junction diode and the gate withstand voltage of the MOS transistor. When the margin is thus reduced, there is a possibility that the MOS transistor is not protected by the PN junction diode. If a significantly large breakdown current flows in the PN junction diode, the voltage across the PN junction diode becomes higher than the gate withstand voltage of the MOS transistor because of the presence of a small resistance of the broken-down PN junction diode. In this way, the MOS transistor may be damaged.